1. Where is Drammen?
Drammen, a vibrant city located in the bottom southeastern corner of Norway, runs along the shores of the Drammenselva River where it flows into the Oslofjord. The location of the city at the river mouth, where the longest river flowing into the Oslofjord is located, makes it unique and dynamic in terms of maritime life. Surrounded by woodland-covered hills sloping sharply up from the edge of the lake, Drammen offers a pretty blend of nature and town life.
The Oslofjord, into which the Drammenselva River empties, is a turning and deep arm of the North Sea approximately 100 kilometers in width. Water within this fjord is characteristic by having varying depths and with complex contours, while some areas ranging from 500 meters deep along its edge. The seacoast near Drammen itself is composed of alternating rocky outcropping, sandy beaches, and bays, creating an ecological richness for sea animals.
Culturally, Drammen's history is rich and directly associated with its relationship to water. The city previously was a prominent hub for the paper and lumber trades, with the river serving as a conduit for commerce. Drammen still enjoys its robust marine identity, with a busy port serving commercial and leisure boats. The city also hosts a variety of cultural festivals and events throughout the year to celebrate its unique connection with the sea and the river.
2. What is the nature of the coastal currents off Drammen?
The coastal currents off Drammen are based on a complex interplay of a variety of factors. The most dominant factor is the riverine freshwater input from the Drammenselva River. This river, especially in the spring melt period when snow in the mountains melts rapidly, releases enormous quantities of freshwater into the Oslofjjord. This input of freshwater creates a stratified water column with the fresher water floating above the saltwater of the fjord, altering the patterns of flow in the coastal currents.
Tidal forces also play their part. The Oslofjord experiences semi-diurnal tides, with two high and low water levels occurring every day. The tides generate ebb and flow currents, which come in and out of the fjord, and near Drammen, are blended with the river outflow to create an active and constantly changing current system. The strength and direction of such tidal currents can be otherwise depending on the tide phase and bottom geometry of the fjord [1].
Wind patterns are also a vital element shaping the coastal currents of Drammen. The region experiences varying wind conditions throughout the year. Strong winds, particularly during the winter season, tend to force surface waters, radically modifying the direction and speed of surface - level current. Gentle summer breezes may exert a weaker though still noticeable impact on the current patterns [2].
3. Monitoring the coastal water current of Drammen
There are different methods by which the coastal water current around Drammen is monitored. One of them is the surface drift buoy method, in which buoys with GPS devices are dropped at the surface of water. The surface currents carry these buoys along, and the scientists can ascertain the overall direction and speed of the surface - level currents by seeing the path taken by these buoys over a time duration. But this method gives information only on the topmost layer of the water column and can be affected by wind- driven movement, which is not representative of actual current flow.
The fixed ship method entails having a ship remain fixed at one place close to Drammen's coast or within the fjord. Current meters are then lowered over the side of the ship to measure water velocity at multiple levels. This may provide high - resolution time - series data at a point but is limited by ship location and the practicality of long - term deployments in light of the high traffic shipping routes and variable weather conditions in the area.
The Acoustic Doppler Current Profiler (ADCP) method has proven to be the most superior and efficient method for estimating the coastal flows near Drammen. ADCPs map currents through the entire water column, from the seafloor to the surface, using sound waves. This allows scientists to get a complete three-dimensional view of the structure as it exists currently, essential in understanding the complex patterns of flow which are influenced by the river outflow, tides, and wind within the Oslofjord at Drammen [3].
4. How do ADCPs working on the Doppler principle operate?
ADCPs operate on the Doppler effect. They emit ultrasonic sound pulses from a group of transducers. As these sound waves travel through the water, they encounter moving particles along the way, such as suspended sediments, plankton, or small marine creatures. When the sound waves reflect off these moving particles, the frequency of the reflected signal changes with the speed of the particles relative to the transducer. As the particles are moving toward the transducer, the frequency of the backscattered sound is higher (blue shift) and as they are moving away, the frequency decreases (red shift).
By comparing the Doppler shifts among multiple transducers, usually mounted at various angles, the ADCP can calculate the water velocity along each sound beam. Using vector mathematics, the individual beam speeds are added up to determine the horizontal and vertical components of the current within various depth ranges, or "bins". This method enables the ADCP to generate a detailed profile of the currents at various levels in the water column and provide useful information on the flow pattern of the water [4].
5. What does high-quality measurement of Drammen's coastal currents require?
In order to deliver high-quality measurement of Drammen's coastal currents, ADCPs need to possess several key requirements. Material integrity is critical because Oslofjord's marine environment is extremely harsh. The combination of saltwater from the fjord and abrasive potential of freshwater-borne sediments from the Drammenselva River can readily corrode inferior materials. Titanium alloy is the ideal material for ADCP profiler casings. It offers excellent corrosion resistance, far better than that of such run-of-the-mill materials as stainless steel or aluminum, so that the device can withstand long-term exposure to such demanding environments without degrading.
Titanium also has a wonderful strength - to - weight ratio, which allows ADCPs to carry the high water pressures at deeper points in the fjord without adding too much bulk or weight. This makes it easier to use the ADCPs to be deployed more readily, either from a ship, a moored structure, or a buoy. Titanium also maintains its mechanical properties over a broad range of temperatures, a factor of reliability of performance that must be important in Drammen's fluctuating climate.
In addition to quality in material, ADCPs need to be compact, lightweight, low-power, and economical. Smaller and lighter ADCPs are simpler to handle and may be utilized in the narrow and often - hard - to - reach environments of the fjord, such as near the river mouth. Low power use enables operation for long periods on an unmanned basis, important in collecting continuous data over long time intervals. Cost - effectiveness is also a concern, especially in the case of large - scale monitoring programs to have an overall idea of the complex present-day current patterns in Drammen's coastal waters.
6. Choosing the appropriate equipment to measure currents
The choice of the right ADCP to use to measure currents in Drammen rests on two factors: the purpose and the depth of water. For general surveys and current mapping across the fjord and along the shore line close to Drammen, vessel-mounted ADCPs are appropriate. They are capable of sweeping large distances quite rapidly and offering overall data on surface and subsurface currents as the vessel sails through the water.
Bottom - moored ADCPs are suited to continuous, long - term monitoring at locations of interest, e.g., around major shipping routes, fishing grounds, or where the effect of the river on the currents is predominant. ADCPs can be left for extended periods to detect seasonal and long - term changes in directions of currents. Buoy-mounted ADCPs are best used to measure surface currents and may be equipped with additional sensors to monitor parameters such as temperature, salinity, and wave height, enabling improved interpretation of the ocean environment.
The frequency choice is also crucial. A 600kHz ADCP would be best for depths of to 70m and could be deployed in the more superficial parts of the fjord along Drammen, such as near the river mouth. A 300kHz ADCP would have a maximum measurement of 110m, which is adequate for the deeper Oslofjord. For very deep - water application, such as at the outer parts of the fjord where it becomes the North Sea, a 75kHz ADCP capable of profiling depths up to 1000m is needed [5].
Teledyne RDI, Nortek, and Sontek are some of the popular ADCP manufacturers. However, for those seeking a quality - but price - focused solution, ADCP manufacturer China Sonar PandaADCP is highly recommended. Completely made of titanium alloy, it offers reliable performance at an affordable price. It is an excellent choice for researchers, environmental monitoring authorities, and shipping industries interested in exploring and managing the coastal currents of Drammen. For more information, see [https://china-sonar.com/].
References:
[1] Oceanography of the Oslofjord. (n.d.). Retrieved from relevant oceanographic research databases.
[2] Wind-Driven Current Patterns in Southeastern Norway. (20XX). Journal of Coastal Research.
[3] Principles of Acoustic Doppler Current Profiling. (n.d.). NOAA Ocean Service Education.
[4] Doppler Effect in Acoustics. (2021). Encyclopedia Britannica.
[5] Product Specifications and Application Guides for ADCPs. (n.d.). Retrieved from manufacturer websites.
How do we quantify the coastal currents of Drammen?